Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of the Orthogonal Frequency-Division Multiplexing (OFDM) digital modulation technology. OFDM is a special case of FDM, wherein multiple subcarriers carry the information stream and are orthogonal to each other. The orthogonality allows simultaneous transmission on multiple subcarriers without interference from each other. In OFDMA, Multiple Access is achieved by assigning subsets of subcarriers to individual users and thereby allowing simultaneous low data rate transmission for several users.
In most wireless systems, however, multipath is an undesirable common propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. The effects of multipath include constructive and destructive interference, amplifying or attenuating the signal power seen at the receiving antenna. Multipath may also cause phase shifting of the signal. Signal variations in amplitude or phase resulted from multipath are also referred as channel response. If the receiver is able to estimate the channel response, then signal degradations caused by multipath effect can be compensated. Thus, to facilitate channel response estimation, OFDMA systems periodically insert pilot symbols, whose values are known to the receiver, into the transmission signal.
The number and the position of pilot symbols to be inserted into the transmission signal in frequency domain and time domain is referred as pilot pattern. Various techniques exist in prior art for providing pilot pattern design in an OFDM system. For example, US Patent Application No. 2006/0120272 describes a transmitting device for transmitting data symbols and pilot symbols in an OFDM system. As shown in FIG. 1 (prior art), two types of pilot symbols are orthogonal to each other and they are transmitted alternatively in frequency domain as well as in time domain.
US Patent Application No. 2006/0285484 proposes a diamond-shaped pilot pattern to achieve accurate channel estimates for high user equipment speeds while also posses the ability to use substantial pilot energy from succeeding frame with minimum latency. This improved pilot pattern is illustrated in FIG. 2 (prior art).
US Patent Application No. 2006/0209732 describes a pilot pattern design method. As shown in FIG. 3 (prior art), the proposed OFDM wireless system can change pilot patterns based on frequency selectivity and Doppler shift information.
US Patent Application No. 2007/0195688 describes spatial pilot structure for multi-antenna and multi-layer transmission wireless communications systems. In one example, a single-layer pilot pattern is extended to multi-layer pilot pattern for multi-input and multi-output (MIMO) receivers.
In OFDMA wireless systems, a resource block is defined as a two-dimensional block comprising a number of consecutive sub-carriers (also referred as frequency tones) by a number of consecutive OFDM symbols (also referred as time slots). IEEE 802.16m defines 5-symbol resource block as 18 sub-carriers by 5 OFDM symbols, 6-symbol resource block as 18 sub-carriers by 6 OFDM symbols, and 7-symbol resource block as 18 sub-carriers by 7 OFDM symbols. They are the smallest unit that can be assigned to a user of a mobile station (MS). Small sized resource blocks have better real time characteristics and they are targeted for VoIP or other small-packet application.
Pilot patterns proposed in the prior arts, however, are designed without consideration of the resource block size. Thus, in OFDMA systems, it is desirable to address optimal pilot pattern design based on predefined resource block size.